1. Field of the Invention
This invention relates to an improvement in thermal recording instruments of the type in which the trace of an analog signal and alphanumeric information are recorded on an incrementally advanced heat responsive web; and, more particularly, to an improved instrument which overcomes the problem of the printhead sticking to the web.
2. Description of the Prior Art
There are a number of instruments in the prior art which record an analog trace and print alphanumeric information concerning this trace on a thermally responsive web. One such device is disclosed in my co-pending application entitled "Dwell Interval Printing," Ser. No. 417,625, filed Nov. 20, 1973, now abandoned. In this device, the heat responsive web is moved incrementally by means of a stepper motor and a solenoid actuated printhead is brought into contact with the web when it is stationary in order to print alphanumeric information. While generally satisfactory, this device has inherent limitations associated with the repeated mechanical motion required of the printhead.
In another prior art instrument for thermally recording analog and alphanumeric information, a synchronous or other similar motor drives the heat sensitive web continuously. U.S. Pat. No. 3,754,279 illustrates such a prior art device. Again, while such a device is generally satisfactory, it, too, has limitations. For example, it is difficult and costly to use a synchronous type motor to drive the web in a controlled manner over the very wide speed variations required of such recorders.
One proposal to solve the problems of the prior art devices is to use a stepper motor to drive the web incrementally and leave the printhead in continuous contact with the heat sensitive web. While this proposal overcomes the limitations of the prior art devices, it has not proved satisfactory because the printhead tends to stick to the web causing distortions in the analog trace.